Stillwater Bridge, MN
Typical DOT Reinforcement Steel Specifications
Transportation authorities have gone through a similar analysis as presented in the decision framework in determining which structures require stainless, the structural elements to be protected and the type of stainless steel material to use. The various criteria upon which some U.S. State departments of transportation (DOT’s) have based their decisions relating to the use of stainless steel rebar are provided.
RECENT USE OF STAINLESS STEEL
MAJOR PROJECTS
SOURCE: Salit Specialty Rebar
Virginia Department of Transportation (VDOT)
VDOT designates three classes of roadway systems based essentially on traffic volume which they call “Functional Classifications”. In parallel, they have established three classes of corrosion resistant reinforcement, all of which are non-coated steel materials. VDOT planners and engineers have designated the use of stainless on the two most important roadway classes in terms of traffic – Classes II and III. The DOT specifies all of the stainless rebar alloys listed in ASTM A955.
VDOT indicates the specific structural elements where the class types of corrosion resistance reinforcement (CRR) material are to be used in the roadway classes.
For example, VDOT designates the use of stainless steel reinforcement in Class II and III roadways in the following structural elements :
Deck slabs
Medians, barriers, sidewalks, terminal walls
Approach slabs and retaining walls
Integral backwalls of abutment
Diaphragms
Reference document: Virginia Department of Transportation, Structure and Bridge Division, Instructional and Informational Memorandum, Corrosion Resistant Reinforcing Steels, IIM-S&B-81.7, December 12, 2016
Patroon Bridge, New York
Vermont Agency of Transportation
VTrans engineers classified their roadway system into three levels on the basis of importance, i.e. traffic volume, or where an extended service life is desirable due to the high cost/difficulty of maintenance.
For the purpose of achieving the intended service life, they established three levels of corrosion resistant reinforcing steels with stainless steel reinforcement included in two of the three levels.
Stainless steel reinforcement is designated in the two most traffic bound roadway classes which includes structures in interstate highways, national highways and town highways. The Agency specifies all of the stainless rebar alloys listed in ASTM A955.
Other lesser corrosion resistance coated reinforcement is to be used on low traffic volume, unpaved roads and for components with a design service life of 30 years or less.
Reference document: Vermont, VTrans, Structures Engineering Instructions (SEI), SEI 12-001, March 15, 2012
Minnesota Department of Transportation (MnDOT)
MnDOT adopted a policy of using stainless steel reinforcement in their bridge decks based on performance, constructability and cost saving characteristics.
Their studies indicate that using stainless steel reinforcement in their bridge decks results in a life cycle cost savings of 10-20% relative to epoxy coated reinforcement. The service life of a bridge deck built with stainless steel reinforcement is expected to last 100 years which is 40 to 50 years longer than with the coated steel option. The DOT cites that though there is an additional initial cost (3 to 5%), the replacement cost is significantly more expensive. Also for some bridges because of their design (box girders) or others because of high traffic volumes, deck replacement would be very problematic.
The criteria for selecting candidate structures where stainless must be used are :
Bridges with superstructure types that prohibit deck replacement such as box girders,
Any bridge with a construction cost exceeding $25 million (2017 dollars), and
Situations of a long detour length, difficult construction staging, high traffic volumes, or undesirable consequences if the bridge deck service life would be less than 75 years.
MnDOT specifies the use of all types of stainless steel reinforcement listed in ASTM A955, or that meets this standard, for use in the following bridge structure elements.
Bridge decks
Medians and barriers
Reference document: Minnesota Department of Transportation, Engineering Services Division, Technical Memorandum No. 17-02-B-01, February 22, 2017
Alberta Transportation
Confronted with the high cost of premature deterioration of bridge components caused by de-icing agents, the DOT has refined its concrete reinforcing standards emphasizing alternative non-coated reinforcing steel materials with improved corrosion resistance in order to reduce maintenance costs. Stainless steel rebar is a primary material that Alberta uses in its bridge structures to reduce the life cycle and user costs.
Alberta Transportation selects the type of improved corrosion resistant material on the basis of the structure’s exposure class, which they relate to the degree of importance they attribute to the structure, and to a life cycle cost analysis. The selection of which structures are candidates for the use of stainless steel rebar is set out on two charts plotting bridge area and traffic volume -- one for undivided highways and another for divided highways. Each chart is divided into exposure class zones with stainless designated for Zone 1 -- the conditions of higher traffic and larger bridge surface area.
Alberta Trans provides the designer with a choice of three specific types of stainless steels listed in ASTM A955 and designates the use of stainless steel reinforcement in Zone 1 structures for the following structural elements of bridges :
Deck slabs
Curbs and barriers, medians, sidewalks
Approach roof slabs, approach slabs
Beams, pier caps
Tops of abutment walls, diaphragms, wingwalls
Deck joint blockouts
Reference Document: Bridge Structures Design Criteria, Technical Standards Branch, Alberta Transportation, April 2017
Performance Guidance in Temperature Extremes
Austenitic types of stainless steels have excellent toughness at the low temperatures prevalent in cryogenic applications such as LNG applictions. Duplex types of stainless steel are generally not suitable in this application because they exhibit brittle behavior at temperatures below 50°C.
Performance Guidance for Magnetic Permeability
In applications requiring a non-ferromagnetic response typified by a very low or negligible response to electromagnetic fields, austenitic stainless steels are suitable. A limit is then usually specified on the relative magnetic permeability of the steel.
Austenitic steels can be classed as 'paramagnetic' with relative permeabilities approaching 1.0. These low permeabilities enable austenitic steels to be used where 'non-magnetic' materials are required. Such applications include concrete reinforcing bars for MRI’s, radar installations, naval facilities.
The permeability of duplex stainless steels is quite different from austenitic stainless steels. Duplex stainless steels are usually classified as 'magnetic' since they exhibit a strong response (or pull) to a hand-held magnet.